What is the importance of the term glass ceiling?

It may be a ceiling made of glass.
However, in most cases it is used to describe a level in an organisation above which (for some reason - gender, class, etc) the 'normal' person can not rise despite performance or ability.
The connotation "glass" means that this barrier can not be seen, it is invisible (but it is there). It is present in most organisations and is a covert form of corruption.